Let say my domain is mydomain.net.I want to send email from user_b@mydomain.net to user_a@mydomain.net, where user_a mailbox in "Mail server on ISP" only and user_b is in "Mail server on ISP" and iredMail server. And I got error "SMTP Error: SMTP error: Failed to add recipient user_a@mydomain.net", if I'm add user user_a in iRedMail, it will deliver to local server (iRedMail server).I've try to set relayhost to "Mail server on ISP" , still not working. Can anybody advise me how to do it?

Let say my domain is mydomain.net.I want to send email from user_b@mydomain.net to user_a@mydomain.net, where user_a mailbox in "Mail server on ISP" only and user_b is in "Mail server on ISP" and iredMail server. And I got error "SMTP Error: SMTP error: Failed to add recipient user_a@mydomain.net", if I'm add user user_a in iRedMail, it will deliver to local server (iRedMail server).I've try to set relayhost to "Mail server on ISP" , still not working. Can anybody advise me how to do it?